Global Changes

In the Sightings column “Watching Earth Change” in the March–April issue, the two figures showing the change in global distribution of atmospheric carbon dioxide are confusing and misleading.

The legend points out that the scale for 2013 has been shifted by 20 parts per million. At first glance, this seems like a trivial adjustment. On closer examination, I discovered that because of this change the scales for 2003 and 2013 are totally different. There is no overlap.

The result is that the big increase in CO2 between the two years is obscured. Only by the closest examination can the actual difference be ascertained. Yes, as the legend says, “the rise in CO2 concentration is visible,” but just barely.

These figures as presented are certainly misleading and miss the opportunity to display a dramatic effect.

John E. Douglas
Spokane, WA

Editor’s Note:

It is correct that the rise in global atmospheric carbon dioxide is barely visible in the pair of maps on page 99 in the March–April Sightings column, but showing overall CO2 concentrations was not the point of the illustration. Scientists at the Jet Propulsion Laboratory, who created the images, intentionally altered the scale to highlight the specific locations where CO2 consistently collects in Earth’s atmosphere. If they had not done so, the entire 2013 map would have been featureless red, with all locations at the top of the 2003 scale bar; the pattern of CO2 distribution, which is useful for studying how the greenhouse gas gets distributed through the atmosphere, would not be visible. To alert readers, we noted the scale adjustment in our caption accompanying the image pair.